The present invention relates to plastic fencing components. In particular, the invention relates to elongated tubular plastic fencing components, e.g., posts, pickets and rails used in modular plastic fencing systems, and improved methods for manufacturing the same.
Plastics present a desirable alternative to wood as a construction material for fencing, particularly in terms of ease of use and durability. For example, modular vinyl (PVC) fencing is generally easy to assemble, highly resistant to the elements and does not require painting. The use of plastics for fencing components is known, e.g., in the agricultural, residential and home improvement industries. Exemplary plastic fencing structures are disclosed in U.S. Pat. Nos. 5,421,556 (Dodge et al.)(commonly assigned); 5,303,900 (Zulick, III et al.); 5,215,290 (Khalessi); 4,727,702 (Baker et al.); and 3,728,837 (Kiefer, Jr.).
The conventional technique for manufacturing tubular plastic fencing components involves the use of profile extrusion, i.e., the extrusion of molten plastic material into the desired tubular profile. At least three significant problems exist with this technique. First, the flow characteristics of plastics materials such as PVC require that the extrusion be performed with relatively low throughput, e.g., 12-15 feet per minute (fpm). Obviously, if the throughput could be increased, production costs would be reduced, thus making plastic fencing an even more viable substitute for wood.
Secondly, profile extrusion of tubular structures inherently requires that certain minimum wall thicknesses be maintained to prevent collapse or warpage of the extruded plastic material while it is still soft. This results in unnecessary material usage, and expense, since the wall thicknesses will be dictated by the production process rather than the requirements of the product's end use. Since, for many fencing applications, adequate strength can be afforded with significantly thinner walls than are required by profile extrusion methods, there is a need for an alternative manufacturing method that would allow formation of lesser wall thicknesses.
Finally, an important concern for many potential purchasers/users of plastic fencing is whether the fencing has a natural appearance, i.e., whether it closely simulates a natural wood fence. Toward this end, it is desirable to provide plastic fencing with a simulated wood grain, and color variegation. However, it is difficult to impart such attributes to tubular fencing components produced by profile extrusion.
The formation of pipe and other tubular structures from sheet metal is known. Typically such processing involves the use of stamping, rolling and/or die-forming to create a generally tubular shape having a pair of adjacent or overlapping longitudinal edges. Some form of crimping or separate connectors are generally used to secure the longitudinal edges to each other to finish the tube. Examples of such techniques are disclosed in U.S. Pat. Nos. 1,796,015 (Francis et al.); 1,034,483 (Mills); and 207,606 (Ketchum).
Heretofore, such forming processes have not provided a viable substitute for profile extrusion in the production of plastic fencing. Principally, what has been lacking is a satisfactory means for producing an aesthetically acceptable and structurally sound longitudinal edge joint as part of a relatively high-speed continuous production process.